Conventionally, the technology of optical lithography comprising transferring a pattern on a photomask to a wafer by using a laser radiation has been widely used in the aligners for producing semiconductor integrated circuits because of its advantage in process cost as compared with other technologies using electron beam or an X-ray.
Recently, as the LSIs increase their fineness and the degree of integration, light sources having shorter wavelength are being used for the exposure, and there have been practically used an aligner using an i line (having a wavelength of 365 nm) which enables the formation of patterns 0.4 to 0.5 μm in pattern line widths, or a KrF excimer laser (emitting a radiation 248.3 nm in wavelength) which enables patterns 0.25 to 0.35 μm in pattern line widths. More recently, an ArF excimer laser (emitting a radiation 193.4 nm in wavelength), which enables the formation of patterns 0.13 to 0.2 μm in pattern line widths, has been developed, and the study to bring it in practical use is under way. However, the optical members for use in the ArF excimer laser lithographic apparatuses demand that they satisfy, at a never required high level, a further increased uniformity, high transmitting properties, an excellent resistance against laser radiations, etc.
As a material for an optical member satisfying the requirements above, a synthetic quartz glass of high purity is being used, and improvements in the optical transmittance and the resistance against laser radiations of such a material have been made by optimizing the production conditions, and, at the same time, a further improvement in optical characteristics such as uniformity and birefringence is being made. Among them, the improvement in uniformity and the reduction of birefringence can be realized only by applying a heat treatment (annealing treatment) accompanying a gradual cooling in the production process of the optical member to thereby remove the stress of the quartz glass. As such a heat treatment, a method comprising holding the quartz glass inside the heating furnace at a high temperature for a long duration of time has been believed to be a general method.
However, on lowering the temperature during the annealing treatment above, temperature distribution generates between the central portion and the outer peripheral portion of the object being treated. Such a temperature distribution remains as a difference in density even after the completion of the annealing treatment, and this led to an insufficient improvement concerning the distribution in refractive index and the birefringence.
Accordingly, in order to further improve the distribution in refractive index and the birefringence of a quartz glass, there has been proposed a method of applying the annealing treatment to the object while placing it inside a ring, a vessel, or a powder; the aim of which being controlling the temperature distribution by decreasing the rate of lowering the temperature for the outer periphery of the object. This method can surely improve the distribution in refractive index and the birefringence of a quartz glass to a certain degree, but the effect was found still unsatisfactory.
A blank of the generic type and a method of producing the same are known from EP-A 401 845. The production of a lens for a microlithographic device is described therein. To this end a rod-shaped ingot of synthetic quartz glass is cut down into a number of plate-shaped blanks, an optical member being normally obtained from each of the blanks. In comparison with the outer contour of the optical member to be produced, each of the blanks is provided with an overdimension which is removed in the course of the further manufacturing process.
The homogeneity of the quartz glass blank depends on both a uniform chemical composition and a homogeneous distribution of the so-called “fictive temperature” across the blank. The fictive temperature is a parameter which characterizes the specific network structure of the quartz glass. A standard measuring method for determining the fictive temperature on the basis of a measurement of the Raman scattering intensity at a wave number of about 606 cm−1 is described in “Ch. Pfleiderer et al.: “The UV-induced 210 nm absorption band in fused silica with different thermal history and stoichiometry”; J. Non-Cryst. Solids 159 (1993) 145–143”.
To reduce mechanical stresses within the plate-shaped blank and to achieve a homogeneous distribution of the fictive temperature, the blank is normally annealed with great care. EP-A 401 845 suggests an annealing program in which the blank is subjected to a holding time for 50 hours at a temperature of about 1100° C. and is subsequently cooled in a slow cooling step at a cooling rate of 2°/h to 900° C. and then in a closed furnace to room temperature. During such a temperature treatment local changes in the chemical composition of the blank, in particular in the areas near the surface, may occur because of an outdiffusion of components. In this respect a particularly long annealing time of the blank for setting a distribution of the fictive temperature that is as uniform possible may even have a disadvantageous effect on the homogeneity of the blank.
The surface of the known blank is defined by an even lower side, an even upper side opposite thereto and by an outer cylindrical surface which connects upper side and lower side. The surface surrounds the contour of the member with an overdimension. An increase in the overdimension alone does not constitute a preferred measure for reducing outdiffusion from the area of the contour of the member during annealing, for larger dimensions of the blank require longer annealing times to ensure a uniform distribution of the fictive temperature within the contour of the member. Longer annealing times increase the manufacturing costs, which in turn promotes outdiffusion. Moreover, a large overdimension entails higher manufacturing costs because of larger losses in material.